Boiler with fusible metal actuated control switch



g- 1953 A. COHEN ET AL BOILER WITH FUSIBLE METAL ACTUATED CONTROL SWITCH Flled Aprll 19, 1951 THERMOS TflT.

Patented Aug. 25 1953 BOILER WITH FUSIBLE METAL ACTUATED CONTROL SWITCH Arthur Cohen, Brooklyn, and Patrick S. Foster, Astoria, N. Y.

Application April 19, 1951, Serial No. 221,843

3 Claims. (Cl. 122-504.3)

This invention relates to furnace controls and in particular to an improved automatic cut off for an electrically operated furnace.

An electrically operated furnace, as it is commonly understood, comprises a boiler, a fire box having a heat source therein and a control regulating the operation of the heat source within the fire box. The control may be a thermostat in a room to be heated or a thermostat responsive to the temperature of hot water to be heated by the furnace.

The boiler is a container housing a fluid, usually water, which is heated to supply a high temperature thermal agent to an area to be heated. For example, the boiler may be a double-walled water container positioned in thermal contact with the fire box. The interwall water-containing spaces are called cores, and they are interconnected to permit the circulation of water, which after being heated is distributed either in the liquid stage for. a hot water heating system, or in the vapor stage for a steam heating system. In effect, the hollow cores form a water jacket which cools the inner wall of the boiler. If heat were not conducted away from the inner boiler wall by the water jacket, hot spots would develop to produce expansion conditions that would crack the boiler wall.

A fire box in a common form is a hollow cylinder or cube having a periphery of fire insulating material, such as'fire brick or fire clay, and a boiler would be positioned on top of the fire box to permit both the radiation from the fire box and the combustion gases generated in the fire box to heat the boiler walls.

A heat source within the fire box may be supplied by burning coal, oil, or gas. Of the electrically operated furnaces, however, the oil-fired is one of the most common. In such a furnace, an electric motor operates both a fuel pump and a blower to inject a fuel-air mixture into the fire box. The operation of this motor may be controlled by a room thermostat, or other temperature-actuated devices, whereby the heat source is supplied to the fire box in response to a signal generated by the temperature-actuated device.

To protect boiler walls from cracking, many controls have been developed to open valves or to interrupt circuits in response to fioat levels, etc., but there is still need in the art for an improved positive furnace control, and it is an object of this invention to provide an improved furnace control that will interrupt an electric circuit to a furnace when the water supply in the boiler falls to a dangerous level.

It is a further object of this invention to provide an improved low cost furnace over-tempera.- ture protective device requiring a minimum of maintenance. I

It is a still further object of this invention to provide an improved furnace control that will thus maintaining it at a safe temperature.

shut off a furnace when a boiler wall is heated above a predetermined temperature and that will maintain the furnace in the shut-down state until a readily replaceable element has been installed in the control.

Further objects and advantages of this invention will become apparent and the invention will be more clearly understood from the following description referring to the accompanying drawing, and the features of novelt which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Briefly, this invention comprises an electrical circuit-opening device positioned in the main line of an electrically operated furnace to be responsive to the temperature of a boiler wall to interrupt the electrical power supply to a furnace when the wall temperature rises above a predetermined or safe level.

While this invention as hereinafter set forth relates to a particular type furnace and to a particular installation, it is to be understood that such a description has been chosen merely as an example to point out more clearly the operating features of this invention, and it is not to be considered a limitation on its scope.

In the drawing, Fig. l is a schematic diagram showing partially in section an oil-fired electrically-controlled furnace having our new improved control positioned in the main electric line to the furnace; Fig. 2 is a front view of the operating elements of our improved furnace control; while Fig. 3 is a side view of the control.

In the drawing, a furnace I is shown as comprising a boiler 2 mounted on top of a fire box 3 which is oil fired by a burner driven by a motor 5. Motor 5, in turn, is in an electric circuit 6 which is controlled by a room thermostat 1 and by our improved control 8, each of which will interrupt the circuit to motor 5 in response to predetermined signals.

Boiler Zis shown as comprising a dual-walled structure having a plurality of water containing interconnected cores 9 therein. As the water passes through the cores 9 of the boiler, it conducts heat away from the inner boiler wall 2a, y this we mean that the temperatures existing within a fire box of a water-jacketed furnace are such as to produceserious expansion problems, if the inner wall 2a is not properly cooled. For this reason, a predetermined water level must be maintainedwithin the cores 9 of the boiler.

Burner 4 in a conventional form has a fuel pump and an air fan driven by motor 5. An ignitor is used to initially fire the fuel air mixture supplied by the pump and air fan to the fire box. That is, the ignitor is employed until the fire box gets up to operating temperature, whereupon the injectedair fuel mixture will burn spontaneously.

When the circuit 6 to motor is interrupted, the fuel pump and air blower stop, thereby extinguishing the flame within the fire box. A conventional means of regulating the fuel supply to a furnace, and consequently its heat output, is to position a switch in circuit 5; the opening or closing of the switch regulates the fuel supply to the furnace. An example of such a switch is relay l1] activated by a coil II. The coil II is in a low voltage secondary circuit la, including the control 1, which might be a bimetallic thermostat responsive to room temperature. Control I actuates coil 1 l, which in turn picks up relay It to complete circuit 6.

Another form of control 7 could be a water temperature actuated thermostat that would be responsive to the water temperature within the boiler cores 9. This control, too, could, through its own secondary circuit lb actuate coil M to complete the primary circuit 6 by closing relay [0. The water temperature actuated thermostat commonly is connected to a tube l2 having one open end positioned in the core 8 of the boiler just below the water level.

Our improved control or over-temperature protective device 8 is not operated by a secondary circuit, but rather is inserted in the primary circuit 5 to provide a switch 43 in series with the relay Hi. This improved over-temperature device is responsive to the temperature of the furnace wall in a manner hereinafter described to make or break a circuit between the terminals Ma and Mb on either side of switch it.

Since our improved control is responsive to boiler wall temperature, it is preferably positioned in intimate thermal contact with the boiler wall at a point considered a danger level for the water in core 9. Then when the water level in core 9 falls below this point, our control is actuated by the high degree of heat in the boiler wall above the water level.

While we do not intend to limit our invention by specifying any particular thermal contact with the furnace wall, it has been found in practice that a convenient way of mounting our new control is to secure it to the tube 52 connecting the water temperature control to the core 9 of the boiler. However, other means of mounting would serve equally as well; for example, a portion of the furnace insulation could be scraped ofi, whereupon our control could be welded, soldered, or otherwise secured to the furnace wall.

In one embodiment, our over-temperature protective device 3 comprises a combined signal bar and support 15, a panel IS, a fusible plug IT, a mercury tube switch 18 and a cradle I!) mounted for rotation about a bearing pin 25.

The combined signal bar and stand l5 includes a clamping portion 2| which may be secured in any conventional manner, such as by U bolts, to the tube [2 of the hot water control. Ideally, the signal bar [5 is made of a material having high heat conduction properties, of which copper would be one example. Obviously, the shape and dimensions of the signal bar l5 are not to be a limitation on this invention, but one convenient form is to employ a vertically extending bar having dimensions in the order of an eighth of an inch in thickness, a half inch in width, and six inches in height.

At the upper end of the signal bar [5 the fusible plug H is connected by a thumb screw 22 passing through an aperture 23 in the fusible plug to engage a threaded bore 24 in the copper bar. This simple interconnection of the elements is of importance since it provides for the ready interchanging of the fusible plugs after one has been melted.

The fusible plug H in one embodiment comprises a cylindrical disk having a diameter of about one inch and a length of about one-eighth inch. These dimensions are not critical, and the plug conceivably could be square or rectangular. The fusible plug, in order to provide for the safe operation of our control, is made of alloys having a low melting point, preferably below 250 degrees F; To this end, we have found suitable an alloy compounded from commercially available solders, such as the tin, lead, bismuth solders having melting points about the boiling point of water. An example of such an alloy is one having 24,8 per cent tin; 22.1 percent lead; 53.1 percent bismuth. This alloy has a melting point of 250 F. and has adequately served the purpose. Mercury tube switch I8 can be one of those commercially available cylindrically shaped glass tubes containing a pair of electrodes l8a, I8b each having a pigtail passing through the glass to provide external circuit connections. A pool of mercury is then positioned within the tube to make electrical contact between the electrodes when the tube is tilted to the degree where both electrodes are in the mercury pool.

To employ the melting of plug ll as the activating instrument for interrupting the circuit 6, we position signal bar [5 in juxtaposition with a panel [3 made of any fiber board or other insulating material, such as those commonly em.- ployed in electrical panels. Panel I5 is provided with a bearing hole Zfia to support bearing pin 2-5, which, in turn, serves as a pivot point for the cradle 59. Any conventional means can be employed for securing bearing pin 20 to the panel. For example, pin 28 can have a threaded end which passes through the bore 23a to be engageable by a nut, not shown, on the opposite side of the panel.

Cradle l9 comprises a plate 25, a. bushing 26 and a pair of U-shaped spring elements 27,, 28 each having a pair of upwardly extending arms which are biased towards one another. Plate 25 has dimensions compatible with the size of the elongated mercury tube [8 which it is to support. Bushing 26 can either be a separate piece secured, as by welding, to one end of plate 25, or it can be formed by bending one end of plate '25. Bushing 2'6 has an internal diameter that will fit oven pin 28 to provide bearing support. The spring elements 21, 2.8 are formed to resiliently hold the mercury tube [8 in fixed engagement with plate 2 5.

The positioning of the pivot pin 25d and its supporting bore 260. relative to the fusible ele ment i'i' is critical in that cradle l9 must be tilted downwardly towards its bushing 25 for operation of tube l8. In this position, the pool of mercury within the tube establishes electrical contact across the electrodes iiia, IB-b which are connected one to each of a pair of pigtails 2.9 and 3&1. To provide for the proper downward tilting of the tube iii, the fusible plug ll is positioned under plate at some point removed from bushing 26. The fusible plug ll, however, must be so positioned relative to pin 2 3 that when the plug melts, cradle [9 will rotate, under the influence of gravity or other biasing force, to cause the mercury to leave the end of the tube containing the electrodes, thereby interrupting the circuit across them. The relative positioning of the pivot pin 20 and the fusible plug ll with its supporting thumb screw 22 is obviously a purely mechanical selection dependent upon the size of the plug l1 and the degree to which the mercury tube I! must be tilted for completion and interruption of the circuit through the electrodes. In the particular embodiment shown, plate 25 is provided with a finger 25a which actually contacts fusible plug IT. The melting of plug 57 rotates plate 25, finger 25a and tube It about pin 20 to open the circuit.

Pigtails 29 and 3|], extending from mercury tube l8, are connected to terminals 3| and 32, respectively, positioned on panel l3. Terminals 3| and 32, respectively, are then electrically connected by leads 33, 34 to terminals Ida and Hit of the switch I3. With this structure, a circuit is completed across terminals Ma and I l-b when the mercury tube I8 is tilted to the position shown in Fig. 1. That is, lead 33 connects terminal I la to terminal 3|; pigtail 29 connects terminal 3| to an electrode in the mercury tube l3; the pool of mercury connects the two tube electrodes; pigtail 3|] connects the second tube electrode to terminal 32 on panel Hi; and lead 34 connects terminal 3| to terminal Mb, thereby establishing the circuit across terminals Ma and Mb.

The advantage of this circuit lies in the fact that the primary circuit 6 is interrupted when the fusible plug H is melted, thereby stopping motor and hence the supply of fuel and air to the fire box 3.

In operation then, any lowering of the water supply in core 9 below a predetermined level will cause a heating of the boiler walls, which will be transmitted by tube l2 to the signal bar I5. The heat is then conducted along signal bar I5 to the fusible plug I! which melts, tilting the mercury tube l8 to interrupt the circuit across terminals Ma and MI), thereby stopping motor 5 and the fuel supply to the furnace.

To facilitate the conduction of the heat from the fire box 3 and tube |2 to the fusible plug l1, it is advisable to provide an insulated cover 35 which can fit over the panel I6 and most of the signal bar Hi to prevent drafts from cooling the signal bar. Preferably, an easily removal cover is employed to facilitate access to the new control for the replacement of the fusible plug I1 and for the occasional application of a drop of oil to the bearing pin 2|).

With this structure, we provide a safe, low cost, easily maintainable over-temperature protective device which is dependent only on a supply of heat from a boiler wall for its operation. It eliminates consideration of intricate secondary circuits to interrupt the fuel supply to the furnace. In addition, a serviceman can readily replace the fusible element to reconnect the furnace after he has corrected the cause of the water supply failure.

This improved over-temperature protective device fills a long sought for need in that the simplicity of operation assures the functioning of this control without the danger of rust and corrosion commonly found in float-operated controls heretofore known in the art.

Modifications of this invention will occur to those skilled in the art, and it is desired to be understood, therefore, that this invention shall not be limited to the particular embodiments disclosed, but that it is intended to cover all modifications which are within the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters'Patent of the United States is:

1. In a heating system comprising in combination a boiler having a water wall, a heat source, an electrical circuit for regulating the energization of said heat source, and a control responsive to a predetermined boiler wall temperature to interrupt said electrical circuit and shut off said heat source, said control comprising a mercury switch, a heat conducting support bar, means mounting one portion of said support bar in thermal contact with the boiler wall, means mounting said switch on another portion of said bar for rotation of said switch to an open or closed circuit position, means including the weight of said. mercury switch biasing the same normally to a circuit-open position, a fusible plug, and means mounting said plug in thermal contact with said bar beneath said switch to tilt and position said switch in a circuit-closed position until said plug is melted.

2. In a heating system comprising in combination a boiler having a water wall, a heat source, an electrical circuit for regulating the energization of said heat source, and a control responsive to a predetermined boiler wall temperature to interrupt said electrical circuit, said control comprising a mercury tube switch electrically connected in the electrical circuit, a heat-conducting support bar, a cradle including means for holding said mercury switch, means rotatably supporting one end of said cradle for movement of said switch in a plane parallel to and juxtaposed with one portion of said support bar to provide for the gravity tilting of said switch normally to a circuit-open position, means mounting another portion of said bar in thermal contact with the boiler wall, a fusible plug, means mounting said plug in thermal contact with said bar beneath the unsupported end of said cradle to tilt said switch to a circuit-closed position until said plu is melted.

3. In a heating system comprising in combination a boiler having a Water wall, a heat source, an electrical circuit for regulating the energization of said heat source, and a control responsive to a predetermined boiler wall temperature to interrupt said electrical circuit and shut off said heat source, said control comprising a mercury tube switch, a heat-conducting support bar, means mounting one portion of said bar in thermal contact with the boiler wall, means mounting said switch on another portion of said bar for rotation to an open or closed circuit position, means including the weight of said mercury switch biasing the same normally to a circuitopen position, a renewable fusible plug, screw means mounting said plug in thermal contact with said bar and beneath said switch to maintain the same in a circuit-closed position until said plug is fused, and a heat insulating cover to protect said bar and said plug from cooling air currents.

ARTHUR COHEN. PATRICK S. FOSTER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,735,907 Mitchell Nov. 19, 1929 1,925,352 Twombly Sept. 5, 1933 2,073,268 Ray et al.' Mar. 9, 1937 2,081,948 Mitchel et al June 1, 1937 2,112,038 Lenegan Mar. 22, 1938 2,115,431 Shively Apr. '26, 1938 2,352,664 Stevenson et a1 July 4, 1944 2,362,423 Walsh Nov. 7, 1944 

